1. Field of the Invention
The present invention relates to the ammoxidation of saturated hydrocarbons, and, more especially, to the conversion of alkanes into admixture comprising .alpha.,.beta.-unsaturated nitriles.
2. Description of the Prior Art
The ammoxidation of olefins is well known to this art, particularly of propylene. However, although the saturated hydrocarbons, which are more widely available, are more desirable starting materials from an economic standpoint, it is also well known to this art that these do not have a comparable reactivity in such type of reaction to form, especially, .alpha.,.beta.-unsaturated nitriles.
One of the difficulties encountered in the ammoxidation of saturated hydrocarbons is the requirement for catalysts suitable for dehydrogenating the saturated hydrocarbon under conditions which minimize or eliminate the combustion of the ammonia and/or that of the hydrocarbon while ensuring a reasonable selectivity, either to .alpha.,.beta.-unsaturated nitrile (target compound), for example to acrylonitrile starting from propane, or to enhanced value compounds (aforementioned nitrile and olefin), for example, to acrylonitrile and propylene starting from propane.
U.S. Pat. No. 3,365,482 describes the ammoxidation, especially of isobutane to methacrylonitrile, on a molybdenum-based catalyst deposited onto eta-alumina, doped with antimony at 508.degree. C. starting from a gaseous mixture containing isobutane, air, ammonia and steam (1.0/4.5/1.0/12.5), the selectivity toward methacrylonitrile attaining a value of 49% for a degree of conversion of the isobutane of 22%.
Beginning with a propane/air/ammonia/steam (1.0/4.7/0.67/12.8) mixture, using the same catalyst and at 550.degree. C., the selectivity toward acrylonitrile decreases to 15% for a degree of conversion of the propane of 29%.
French Patent 2,027,238 (corresponding in part to U.S. Pat. No. 3,670,009) describes a process for the ammoxidation, in vapor phase, of saturated hydrocarbons at a temperature greater than 500.degree. C. on a solid catalyst, particularly of tin oxide, boron oxide, molybdenum oxide and silica. Thus, in Example IX of the Table on pages 12-13, the selectivity toward acrylonitrile attains a value of 35%, at 32% conversion of the propane, but under operating conditions wherein the propane/ammonia/air (1/1.2/12) reaction mixture is in the explosive region.
French Patent 2,072,334 (corresponding to British Patent No. 1,336,135) describes a process for the catalytic ammoxidation of alkanes in the vapor phase, at a temperature below 500.degree. C. employing a high concentration of alkane in the gaseous feed mixture, on a solid catalyst, particularly of tin oxide and molybdenum oxide (90/10 by weight); however, better results are obtained using catalysts constituted of antimony oxide and vanadium oxide.
French Patent 2,072,399 describes a process for the catalytic ammoxidation of alkane, in the vapor phase, employing a high concentration of alkane in the gaseous feed mixture, on a solid catalyst which in particular is a binary mixture of oxides containing molybdenum oxide.
The following pairs are more particularly indicated:
(Mo, Sb) (Mo, Sn) (Mo, V) (Mo, Ti) (Mo, Bi).
However, none of these pairs offers better results than those obtained using pairs which do not contain molybdenum. The yields of acrylonitrile obtained are very low; at best, 1.7% of the propane is converted to acrylonitrile at 570.degree. C. using a catalyst based on the oxides of tin and titanium.
French Patent No. 2,119,492 (corresponding to U.S. Pat. No. 3,746,737 and to British Patent No. 1,337,759) describes the use of a binary composition based on the oxides of molybdenum and cerium. However, the results from the pair (Mo, Ce) appear poor in the absence of a halogen or halogenated compound.
It is also described to add to such binary composition (Mo, Ce) a third element selected from between tellurium and bismuth (cf. U.S. Pat. No. 3,833,638). Here again, the results of the catalytic system appear poor in the absence of a halogen or halogenated compound. Moreover, it will be appreciated that in the presence of CH.sub.3 Br, the selectivity toward acrylonitrile attains a value of 67% at a 98% conversion of the propane, but under operating conditions which establish the propane/ammonia/air (1/1.2/12) reaction mixture in the explosive region.
French Patent No. 2,119,493 describes carrying out the ammoxidation of alkanes in the vapor phase on a solid catalyst containing oxides of bismuth and molybdenum and, if appropriate, phosphorus and silica.
Here again, the results of the catalytic system appear poor in the absence of a halogen or halogenated compound and the reaction mixture is placed in the explosive region.
Because of the aforesaid disadvantages and drawbacks, considerable research, whether concurrent or subsequent, has been conducted on the use of solid catalysts based on vanadium and/or antimony.
In Chemistry Letters, 1989 (pp. 2173-2176) the ammoxidation of propane in the gas phase is described, using multicomponent metal oxides containing molybdenum and bismuth and having a structure of the type of that of scheelite. It appears that, despite the relatively moderate temperatures employed, the proportion of combustion products (CO, CO.sub.2) is very high in all instances (at least 15%) and that certain catalytic compositions tested have very little activity with respect to the desired reaction, despite their use under conditions which are in the explosive region or very near such region.
Too, the presence of a halogenated compound can promote corrosion of the apparatus and is thus undesirable in an industrial process.
Moreover, it is apparent that the coproduction of large quantities of CO and CO.sub.2 is also undesirable on an industrial scale.
In addition, employing reaction mixtures which are in the explosive region is even less desirable on an industrial scale when the process is carried out in a stationary bed.